The Sun is not as quiet as it seems, and large or small solar activities caused by solar magnetic field occur every moment. Up to now, all measurements related to the solar magnetic field have been polarization measurements. 

Based on the principle of the Zeeman effect, the magnetic field strength can be simply obtained by measuring the Zeeman splitting of a magnetic sensitive spectral line. However, this is almost inapplicable for visible light and near-infrared spectral lines. 

The Mg I 12.32 and 12.22 μm lines in the mid-infrared are a pair of emission lines that present a great advantage for accurate solar magnetic field measurement due to their long wavelength, making it possible to directly use the Zeeman splitting formula to measure the magnetic field. 

In collaboration with the National Solar Observatory (NSO) of the U.S.A., the team led by Prof. DENG Yuanyong, from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), simulated the formation process of the two lines in different solar atmospheric models in detail. They found that the Mg I 12.32 μm line is more suitable for the solar magnetic field diagnosis and developed a fast new magnetic field calibration technology. 

Their study was published in Astronomy & Astrophysics on Feb. 10. 

Results showed that the relative emission intensity and polarization signal of the Mg I 12.32 μm are much stronger than that of the 12.22 μm line. It is the spectral line with the highest magnetic sensitivity found so far. 

It is also the main working line selected by the Infrared System for the Accurate Measurement of Solar Magnetic Field (AIMS), which is the first astronomical telescope working in the mid-infrared wavelength in China. 

"Our findings will help to better understand future observations from AIMS," said Dr. BAI Xianyong from NAOC, the corresponding author of the study. 

AIMS telescope (Image by HSOS/NAOC)